The present invention relates to heat recovery steam generators and particularly to their water flow circuits. Heat recovery steam generators are used to recover heat contained in the exhaust gas stream of a gas turbine or similar source and convert water into steam. In order to optimize the overall plant efficiency, they include one or more steam generating circuits which operate at selected pressures.
There are essentially three types of boilers as distinguished by the method of water circulation in the evaporator tubes. They are natural circulation, forced circulation and once-through flow. The first two designs are normally equipped with water/steam drums in which the separation of water from steam is carried out. In such designs, each evaporator is supplied with water from the corresponding drum via downcomers and inlet headers. The water fed into the circuits recovers heat from the gas turbine exhaust steam and is transformed into a water/steam mixture. The mixture is collected and discharged into the drums. In the natural circulation design, the circulation of water/steam mixture in the circuits is assured by the thermal siphon effect. The flow requirement in the evaporator circuits demands a minimum circulation rate which depends on the operating pressure and a local heat flux. A similar approach is taken in the design of a forced circulation boiler. The major difference is in the sizes of the tubing and piping and the use of circulating pumps which provides the driving force required to overcome the pressure drop in the system.
In both natural and forced circulation designs, the circulation rate and, therefore, the mass velocity inside the evaporative circuits is sufficiently high to ensure that evaporation occurs only in the nucleate boiling regime. This boiling occurs under approximately constant pressure (constant temperature) and is characterized by a high heat transfer coefficient in the boiling regime. Both of these factors result in the need for less evaporative surfaces. While the cost of evaporators is reduced, the cost of a total circulation system is high since there is a need for such components as drums, downcomers, circulating pumps, miscellaneous valves and piping, and associated structural support steel.
The third type of boiler is a once-through steam generator. These designs don't include drums and their small size start up system is less expensive than the circulation components of either a forced circulation or a natural circulation design. There is no recirculation of water within the unit during normal operation. Demineralizers may be installed in the plant to remove water soluble salts from the feedwater. In elemental form, the once-through steam generator is merely a length of tubing through which water is pumped. As heat is absorbed, the water flowing through the tubes is converted into steam and is superheated to a desired temperature. The boiling is not a constant pressure process (saturation temperature is not constant) and the design results in a lower log-mean-temperature-difference or logarithmic temperature difference which represents the effective difference between the hot gases and the water and/or steam. In addition, since the complete dryout of fluid is unavoidable, in once-through designs the tube inside heat transfer coefficient deteriorates as the quality of steam approaches the critical value. The inside wall is no longer wetted and the magnitude of film boiling is only a small fraction of the nucleate boiling heat transfer coefficient. Therefore, the lower logarithmic temperature difference and the lower inside tube heat transfer coefficient result in the need for a larger quantity of evaporator surface.
To minimize the increase in heating surface, a higher mass velocity is achieved by minimizing the number of the evaporative surface circuits. However, the high velocity required to achieve an appropriately higher heat transfer coefficient results in a higher pressure loss, a higher saturation temperature, and a further lowering of a logarithmic temperature difference. The impact on the surface requirement depends on operating pressure and it is relatively small for higher pressure designs above approximately 400 psig. It has, however, a significant impact on surface selection for a low pressure application below approximately 400 psig, making, in many cases, the once-through design impractical for low pressure application.